Polyolefin composites can be used as such and formed into different articles, such as foam plastic or components in semi-conductor layers on electric cables. Composites are known which consist of polyolefin and a vinyl polymer.
In attempts to improve the consistency, dimension stability, printability and other properties of polyolefins that are formed into different articles or foamed plastics, attempts have been made to mix the polyolefin with a vinyl polymer such as polystyrene. However, because of the low compatibility of the polystyrene in the polyolefins, it is extremely difficult to obtain homogeneous mixtures having an attractive appearance. Even small amounts of 0.2-10 percent by weight, result in a lower impact resistance and poor appearance for the mixture.
Attempts have been made to overcome these problems by polymerizing the styrenes inside the polyolefin particles. There are basically two different methods of polymerizing the styrenes in situ in the polyolefin particles and both of these methods are based on the diffusion of the styrene inside the polyolefin particles, after which the polymerization of the styrene is carried out in conventional manner. The major difference between the two methods is the manner in which the styrene is diffused inside the polyolefin particles.
West German patent DE 29 07 662 describes the polymerization of styrene in situ in several different polyolefins. In this method the styrene is added slowly to an aqueous suspension containing the polyolefin particles at an elevated temperature. The impregnation and polymerization of the styrene proceed simultaneously, resulting in the polymerization tending to happen mainly in the surface regions of the polyolefin particles, with the result that the polystyrene is enriched in the surface layer by the polyolefin particles.
U.S. Pat. No. 4,412,938 describes polymerization of styrene in situ in ethene-vinylacetate copolymers. In this method the entire amount of styrene is added at once to an aqueous suspension containing the ethene-vinylacetate copolymer particles. The addition of the styrene is carried out at such low temperature that no polymerization occurs. It is not until the styrene, or the major portion of the styrene, has diffused inside the ethene-vinylacetate copolymer particles that the temperature is increased and the polymerization begins. A disadvantage of this method is that it takes several hours before the styrene has diffused inside the ethene-vinylacetate copolymer particles so that the method is time consuming.